(1) Field of the Invention
The present invention relates to an electrophotographic developing process and apparatus utilizing an electret. More particularly, the present invention relates to a developing process and apparatus capable of forming a high-density image while preventing occurrence of background fogging.
(2) Description of the Related Art
In a developing apparatus of a commercial electrophotographic copying machine, the magnetic brush developing process is widely adopted For example, a two-component type developer consisting of a mixture of a magnetic carrier and an electroscopic toner or a one-component type developer consisting of a powder having magnetic properties is electrically charged, a magnetic brush of the developer is formed on a developer-delivering member (sleeve) having magnets disposed in the interior thereof, the magnetic brush is moved to the surface of a photosensitive material having an electrostatic latent image, and the surface is brought into sliding contact with the magnetic brush under a bias electric field to form an image.
Furthermore, the developing process using a non-magnetic one-component type developer has already been proposed, and for example, Japanese Unexamined Patent Publication No. 60-136773 proposes a developing apparatus for visualizing a latent image by bringing a developer into contact with or access to a latent image support, in which a developer-delivering member supporting the developer on the surface and delivering the developer along a course including the visualizing region is formed of an electret. In this apparatus, the transfer of the developer is controlled by applying a direct current or alternating current bias voltage between the developer-delivering member and latent image support.
In the conventional developing process, electric adhesion of the developer (toner) to the latent image area (charged area) and prevention of adhesion of the developer (toner) to the non-latent-image area are mainly accomplished by applying a bias voltage between the developer-delivering member and the photosensitive material. In order to obtain the applied bias voltage at the developing step, it is necessary that the developer-delivering member should be disposed in a state electrically insulated from the machine frame. The machine frame is grounded and electricity is applied to the developer-delivering member in this state. However, this is very difficult to accomplish and problems occur not only in the design of the apparatus but also in its operation.
In fact, in practical copying machines, a trouble of insufficient application of the bias voltage is caused by insufficient application of electricity to the developer-delivering member, resulting in occurrence of such troubles as background fogging. In order to prevent occurrence of such troubles, repairing and inspection of surrounding electric parts and members should be performed periodically and indeterminately, which is a serious disadvantage in the maintenance.
Moreover, special parts are necessary for disposing the developer-delivering means in the electrically insulated state so as to apply the above-mentioned bias voltage, and also complicated designing is necessary and transformer parts and other electric circuit parts become necessary. Accordingly, the cost of the apparatus increases and the space for the apparatus increases, and also the weight of the apparatus increases. Thus, problems arise with respect to the hardware.